Milling heads

Old February 13th, 2019, 03:32 PM
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Milling heads

Does anyone know how much can safely be milled from C heads?
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Old February 13th, 2019, 09:15 PM
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How much are you wanting to mill from the heads?

What pistons are you running?
What is your compression ratio target?

What head gasket are you running/planning on?

I would say you would want to do some sonic-checking of the decks of your heads to see how thick they are now before milling them more.....

If your stock C heads start at 80cc's, and you average 1cc reduction for every .006 milled off the heads, you would theoretically be at 70cc's for a .060 cut off the heads, this would make an increase in compression about 3/4 point higher for an average 455 with 9:1 compression using the original 80cc heads.

This is a great calculator for you if you didn't already have it:
https://uempistons.com/p-27-compress...alculator.html


I personally wouldn't feel good about taking more than .090 off of the decks of the head, with or without sonic checking.

Also, depending on what horsepower you are expecting, you don't want to thin the heads down too much on the decks on a higher-horsepower engine, as they won't be stiff enough to hold the combustion pressures without loosing the head gaskets.
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Old February 14th, 2019, 08:09 AM
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Milling heads

I have stock pistons, stock bottom-end. Cranking compression test showed 149 psi. I have the heads off, between milling the heads some and a cam change looking to get that number more like 175-180.
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Old February 14th, 2019, 08:24 AM
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Originally Posted by rekain View Post
I have stock pistons
That doesn't tell us much about the compression ratio without knowing what size and year engine you are working on. C heads were used on 400 (E and G block), 425, and 455 engines from 1967 to 1969.
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Old February 14th, 2019, 08:56 AM
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Milling heads

69 455 originally in a Toronado, now in a 69 442 convertible.

Last edited by rekain; February 14th, 2019 at 03:50 PM.
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Old February 14th, 2019, 05:22 PM
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A leakdown test will tell you far more about how well the rings seal. If the compression is even but low, that could be due to a stretched timing chain, cam profile, etc.

how did you do your compression test? Did you have the throttle wide open? All the spark plugs out? Fully charged battery?
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Old February 14th, 2019, 07:25 PM
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Milling heads

Engine was warm, throttle blocked open, plugs out, battery fully charged.
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Old February 14th, 2019, 10:40 PM
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What elevation are you at relative to sea-level?

or figure in your calculated density altitude...


cranking compression helps to somewhat determine engine health and dynamic effective compression, but isnít as accurate as a leak-down test for seeing if your valves and rings are in good shape

Last edited by Battenrunner; February 14th, 2019 at 10:44 PM.
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Old March 1st, 2019, 03:34 PM
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Too much talk. I went .060 on my Toro heads on a 455 motor. Ran great.
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Old March 1st, 2019, 03:38 PM
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Milling heads

Originally Posted by Gerald Nickels View Post
Too much talk. I went .060 on my Toro heads on a 455 motor. Ran great.
That's what I did. Calculated compression when the heads were off, had something like 8.8-1. Should have 9.6-1 now.
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Old April 27th, 2019, 03:20 PM
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Just flat top the piston for instant compression boost.
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Old April 28th, 2019, 03:22 PM
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The cc measurement you end up with after milling depends on the cc measurement of the head BEFORE milling.

I had my "C" heads planed .030 or .060, and AFTER planing, they measured 83cc on most chambers. What GM claims for chamber volume may not be what you have in real life. Especially if the valves have been sunk into the throats due to heavy valve-seat grinding.

In terms of compression increase, you're usually better-off milling the block rather than milling the head (although with an Olds, the difference isn't near as great as any engine family that has a big quench/squish pad on the head.) Milling the block does tighten-up the quench/squish distance, milling the head does not.
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